Note 4.txt

Class 3

# let’s start by creating an empty file
touch test.txt # all this does is record the filename and file’s starting address on the hard drive; then at that starting address, write an end-of-file (EOF) marker

# ls -l gives us the long listing format for files and directories
ls -l test.txt # here, we say we want to see the listing only for test.txt

# the basic linux security model defines read-write-execute permissions on files and folders for owner, group, and world
# we use the chmod command with two arguments: the first defines read-write-execute permissions for owner, group, and world,
# while the second is the name of the folder or file whose permissions we are setting
# 4, read: 2: write, 1: execute
chmod 444 test.txt # this sets permissions on the file test.txt to read-only for owner, group, and world

# we can verify the change of permissions:
ls -l test.txt

# let’s set permissions back to read-write for owner and group:
chmod 664 test.txt # 6=4 for read + 2 for write

# a bash session has many configuration options. these options are used to set up a bash session before a terminal window is presented to the user.
# three of these options are STDIN, STDOUT, and STDERR, which stand for standard-in, standard-out, and standard-error of a session.
# default standard input device is the keyboard, default standard output device is the terminal window, and the default standard output for errors is
# the terminal window.

echo 'hello, world!' # because STDOUT is the terminal window, echo simply echoes the string to the terminal window

# bash has uses ‘<‘, and ‘>’ as redirection operators. we can use ‘>’ to redirect output to a file.
echo 'hello, world!' > test.txt # this writes the string ‘hello, world!’ to the file test.txt

# the cat command with just one filename as an argument reads the contents of that file to STDOUT
cat test.txt

# this writes a new string to the file
echo 'good-bye, cruel world!' > test.txt

cat test.txt # we see that we have overwritten the contents of the file

# accidentally overwriting the contents of a file is known as ‘clobbering the file’.
# there are ways to keep us from clobbering our files (especially importing when dealing with scripts and datafiles!)
# one way is to remove the write bit from files we want to protect with the chmod command

# when we want to append to the existing contents of a file, we use double redirect operator ‘>>’
echo 'good-bye, cruel world!' >> test.txt

# often we want to see the whitespace characters embedded in strings:
# e.g., we want to know ’is it a space, or a tab?’
# because whitespace characters can be used as field separators in a datafile, this is an important question
# the cat command with the -vet option shows us the normally hidden whitespace characters
cat -vet test.txt # the cat vet sees all!

# we can embed whitespace characters into a string argument in an echo statement by using the -e option:
echo -e 'good-bye,\t cruel world!' >> test.txt 

cat test.txt 
cat -vet test.txt # now we see that a tab is represented by ^I, and a linefeed is represented by $

# when we make a file read-only with the chmod command, we protect it from getting clobbered; but now not only can we not write to the file,
# we also can’t append to it, either

# fortunately, we can set a bash session option that allows us to append to files while preventing us from overwriting them:
set -o noclobber # set the noclobber option

echo 'bye-bye' > test.txt
echo 'bye-bye' >> test.txt
cat test.txt

# we can reset the noclobber option in a bash session
set +o noclob